The invention relates to a method for producing moldings, in particular briquettes, from fine-grained to medium-grained mixed material using organic binders.
While the production of pig iron in blast furnaces with coke uses an artificially produced lumpy carbon carrier as an energy source, reducing means and a supporting framework of a fixed bed, the smelting reduction process based on the COREX®/FINEX® method uses lumpy coal in this function. In the case of commercially available coals, a certain proportion is too fine in terms of grain size to perform the function of a supporting framework in the gassed-through upper part of the fixed bed and in the lower part of the fixed bed that is penetrated by the liquid pig iron and liquid slag. This sub-fraction is therefore separated from the lumpy coal used in the smelting reduction process by screening, it being possible for the screening to be performed before and/or after drying of the coal. The dried sub-fraction of the coal can be transformed into a lumpy form for example by means of briquetting, and consequently made available for being used in a way equivalent to lumpy coal in the smelting reduction process. To obtain a grain size that is suitable for the briquetting, it may be necessary for the screened undersize or coal intended for the briquetting optionally to pass through a crusher before the actual briquetting can be performed. Depending on the type of binder used, the briquettes discharged from the briquetting press usually require subsequent treatment in the form of cooling or heating or a certain dwell time to develop strengths. After that, they are suitable for transporting and bunkering and can be used in a smelting reduction process based on the method described.
The conventional procedure for the briquetting of hard coals with organic binders, such as for example coal-tar pitch (or asphalt bitumen), essentially comprises that the coal is prepared with respect to the grain size and moisture content, followed by the mixing in of a binder with simultaneous use of live steam, to set the required mixing temperature. The mixing is carried out by kneading while feeding in live steam, for instance at temperatures of 90-100° C. The vapor is removed from the mixture in order to reduce the moisture content, with vapors and gasses being drawn off. In a subsequent step, the production of the briquettes is performed.
A particular disadvantage here is that, during the vapor removal, organic pollutants are discharged with the vapor, which is also known as the stripping effect. In the case of coal-tar pitch as the organic binder, the organic pollutants contain compounds that are classified as carcinogenic. On account of their hazardous potential for the operating and maintenance personnel, the use of coal-tar pitch as the binder is greatly restricted or prohibited in Europe (for example TRGS 551 in Germany). In hard coal briquetting (briquettes for household coal), coal-tar pitch has therefore been replaced by asphalt bitumen or molasses.
Unlike in the case of household coal, coal briquettes for use in smelting reduction processes must have not only mechanical properties but also sufficient metallurgical properties, such as for example thermal shock resistance, thermomechanical resistance and low reactivity to CO2.
However, on account of the high alkali content of commercially available grades and the addition of lime that is necessary in this case during the briquetting, prior-art briquettes bound with molasses (such as for example according to WO02/50219, WO/020555 and WO 2005/071119) are extremely unstable with respect to hot CO2 gas. Use of relatively great proportions of such briquettes in a smelting reduction process must therefore be compensated by correspondingly great proportions of lumpy coal with good metallurgical properties and/or metallurgical coke.
Although briquettes produced with asphalt bitumen as the binder generally meet the metallurgical requirements of a smelting reduction process, that is to say they take a mid-range position between briquettes bound with molasses and briquettes bound with coal-tar pitch with respect to their reactivity behavior, this variant of the method is currently not attractive because of high crude oil prices.
In countries with high coking coal production in which coal-tar pitch is available relatively inexpensively but crude oil and molasses are imported goods, there are economic advantages to a particular extent in favor of using coal-tar pitch as the binder.
It must be taken into account in this respect that the briquettes bound with coal-tar pitch have the potential for dispensing with the need for the addition of relatively expensive components, such as metallurgical coke and/or semicoking coal or else coking coal for mixing charge coal.
On the other hand, increased environmental and safety awareness has recently become established even in the developing industrial countries of Asia, with European standards being adapted. In such countries, too, approval for the operation of a briquetting plant with coal-tar pitch as the binder is only possible if the escape of organic pollutants is prevented with certainty.
Prevention of emissions of organic pollutants means that the plant must be of such a configuration that it is largely encapsulated with respect to the environment. Inside the plant there must be negative pressure with respect to the surroundings. The amounts of gas extracted to maintain the negative pressure must pass through wet or dry dedusting and the dedusted gases freed of organic remains by way of subsequent thermal treatment. In the case of wet dedusting, the waste water must undergo appropriate treatment. The filter residues of the waste water purification must undergo proper disposal. However, this is not cost-effectively achievable by conventional methods, because in this case considerable amounts of contaminated condensates or waste water would be produced from wet dedusting facilities.